Chemical vapor deposition method for depositing silicide and apparatus for performing the same

ABSTRACT

A chemical vapor deposition (CVD) method for depositing a silicide and a CVD system for performing the same are disclosed. A silicide is deposited on a substrate. Residual gases remaining from the depositing step are purged out by flowing air including H 2 O (g), to substantially remove fumes caused by the residual gases. In the purge step, the cycle purge is carried out at the conditions similar to the flow of atmosphere, to substantially remove the fumes.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to an improved chemicalvapor deposition process, and particularly to a process for thedeposition of tungsten silicide from dichlorosilane (DCS), and anapparatus for performing the same.

[0003] 2. Description of the Related Art

[0004] As the packing density of semiconductor devices in integratedcircuits (IC) increases, feature sizes of patterns formed on the IC andthe space between the patterns are becoming smaller and smaller.Conventionally, polysilicon has been used for making electricalconnections, such as between a gate electrode and a bitline. However, asthe pattern size and spacing becomes smaller, the RC time delay and theIR voltage drop continue to increase because of the relatively highresistivity of polysilicon. Accordingly, a polycide having similarcharacteristics to those of polysilicon and a resistivity less than thatof polysilicon, has been used as a lower resistance alternative topolysilicon. Often, polycide is a multi-layer structure including arefractory metal silicide (silicide) over a layer of doped silicon.

[0005] Refractory metal silicides may be made from tungsten, molybdenum,titanium or tantalum, all of which have a relatively high melting pointand are useful in the manufacture of VLSI circuits. A silicide may becombined with a highly doped polysilicon to form a polycide gateelectrode. A known method for depositing the refractory metal silicideis by low-pressure chemical vapor deposition (LPCVD). Particularly, incase of using a tungsten silicide combined with a polysilicon, it isknown that characteristics such as self-passivation, stability to wetchemicals, surface roughness, adhesion, oxidation and reproducibilityare excellent.

[0006] Tungsten silicide (WSi_(x)) thin films have been deposited by alow-pressure chemical vapor deposition (LPCVD) method onto semiconductorsubstrates using silane (SiH₄) and tungsten hexafluoride (WF₆) asprecursor gases. One problem with this process is that the depositedfilm does not have a conformal shape over stepped topographies asdesired. Another problem with this process is that films thus depositedmay have a high residual fluorine content that can adversely affectdevice performance. For example, when the semiconductor wafer is exposedto an elevated temperature (e.g., about 800° C. or higher), as duringannealing, the excess fluorine ions can migrate through the underlyingpolysilicon layer into an underlying silicon oxide layer (the gateoxide). This fluorine migration can adversely impact the electricalproperties of the silicon oxide, which in turn can lead to an adversechange in electrical properties of semiconductor devices including suchlayers.

[0007] When using a multi-chamber vacuum processing system according toa known method, a substrate to be coated with tungsten silicide first iscleaned using a fluorine plasma scrub to remove native oxide from thepolysilicon layer. The cleaned substrate is then transferred into asubstrate transfer chamber. This transfer chamber has a nitrogen orargon atmosphere to assist in preventing re-oxidation of the substrate,and contains a robot to transfer the substrate into a processing chamber(e.g., a tungsten deposition chamber) through a slit valve having anO-ring seal. This CVD process has become widely used for deposition oftungsten silicide from SiH₄ and WF₆. However, as substrates becomelarger and feature sizes for devices become smaller, the above problemsof the step coverage and the residual fluorine content may be morepronounced.

[0008] An improved process for depositing WSi_(x) films usingdichlorosilane (DCS) instead of SiH₄ has been proposed. The resultantWSi_(x) films have a reduced fluorine content and are more conformalthan WSi_(x) films deposited using SiH₄ as the precursor gas, therebyproviding a solution to the SiH₄-based deposition process limitations.

[0009]FIG. 1 is a schematic view of a conventional deposition apparatusfor depositing a tungsten silicide from dichlorosilane (DCS). Asubstrate such as a silicon wafer is introduced into the load-lockchamber-A 50 or load-lock chamber-B 52 of a chemical vapor deposition(CVD) apparatus. For example, after loading the wafer into the load-lockchamber-A 50, the pressure of load-lock chamber-A 50 is reduced to about200 m Torr. Thus, the load-lock chamber-A 50 is maintained substantiallyat vacuum. Then, a slit valve (not shown) between a transfer module 54and the load-lock chamber-A 50 is opened and the wafer in the load-lockchamber-A 50 is transferred into a processing chamber-C 56 or aprocessing chamber-D 58 by the robot arm of the transfer module 54.

[0010] When transferring the wafer into the processing chamber-D 58, theDCS deposition process progresses while introducing reaction gases intothe processing chamber-D 58. After the DCS deposition is completed, theprocessing chamber-D 58 is pumped down to a pressure of about 20 m Torr.After completing this pumping step, a slit valve (not shown) between thetransfer module 54 and the processing chamber-D 58 is opened and thewafer is transferred into a cooling stage 60 by the robot arm of thetransfer module 54. After cooling the wafer in the cooling stage 60, theslit valve between the transfer module 54 and the load-lock chamber-B 52is opened and the wafer is transferred into the load-lock chamber-B 52.

[0011] After all wafers are transferred into a cassette in the load-lockchamber-B 52 via the above-described steps, a vent gas such as nitrogen(N2) or argon (Ar) is supplied through a vent line connected with theload-lock chamber 52 until the pressure of the load-lock chamber-B 52reaches a pressure of about 760 Torr. As a result, the load-lockchamber-B 52 is vented and the wafers are removed from the CVD system.

[0012] The reaction gases introduced into the processing chamber duringthe above described DCS deposition process are WF₆ and SiH₂Cl₂ gases.The reaction used to produce WSi_(x) is:

WF₆+SiH₂Cl₂+P→WSi_(x)+by-product gases

[0013] where, phosphorous (P) is contained in the silicon wafer (e.g.,the polysilicon layer under the tungsten silicide film).

[0014] When unloading the wafer from the load-lock chamber aftercompletion of the DCS deposition process, fumes are generated. When thetemperature of the DCS deposition process is 600° C. or more, asubstantial amount of fumes are generated. Further, as the concentrationof P in the underlying polysilicon layer increases, these fumes aregenerated even more. That is, the generation of such fumes is moresevere when the DCS deposition process is carried out on the bitlinepolysilicon layer of which the doping concentration is higher than thatof the gate polysilicon layer.

[0015] As shown in the above reaction, such fumes are generated due tothe by-product gases absorbed on a surface of the wafer after completionof the DCS deposition process. The components of such fumes may bephosphorus-based and/or chlorine-based gases. As a result of analyzingthe components of the fumes, the phosphorous-based gas of about 56parts-per-billion was detected by the phosphine (PH₃) measuring system,and thus, the by-product gases generating the fumes proved to be theP-based gases. Since the P-based and/or Cl-based fumes are noxious tothe human body, the semiconductor process may not be further proceededwithout removing such fumes in consideration of the safety. Accordingly,it is necessary to remove the residual gases absorbed on the wafersurface after the DCS deposition process is completed.

[0016] What is needed, therefore is a CVD method and apparatus whichsubstantially eliminate noxious gases which plague conventional methods

SUMMARY OF THE INVENTION

[0017] In accordance with an illustrative embodiment of the presentinvention, there is provided a chemical vapor deposition method thatincludes depositing a silicide on a substrate and purging residual gasesremaining from the depositing step by flowing air including gaseous H₂O(H₂O(g)).

[0018] According to another illustrative embodiment of the presentinvention a substrate is loaded in a load-lock chamber of a chemicalvapor deposition apparatus. The substrate is transferred into aprocessing chamber. A silicide is deposited on the substrate in theprocessing chamber. The substrate is transferred into the load-lockchamber. Residual gases remaining from the deposition step is purged outby flowing air including gaseous H₂O (H₂O(g)) into the load-lockchamber.

[0019] In accordance with another illustrative embodiment of the presentinvention, there is provided a chemical vapor deposition apparatusincluding a load-lock chamber, a processing chamber mounted on theload-lock chamber, a vent line connected with the load-lock chamber, andan air purge line of flowing air including H₂O (g) into the load-lockchamber to purge residual gases in the load-lock chamber.

[0020] Advantageously, according to exemplary embodiments of the presentinvention, air including H₂O (g) is supplied to purge the residual gasesremaining after deposition of the silicide film, thereby substantiallyremoving the fumes generated due to the residual gases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is best understood from the following detaileddescription when read with the accompanying drawing figures. It isemphasized that the various features are not necessarily drawn to scale.In fact, the dimensions may be arbitrarily increased or decreased forclarity of discussion.

[0022]FIG. 1 is a schematic view of a conventional deposition apparatusfor depositing a tungsten silicide from dichlorosilane (DCS).

[0023]FIG. 2 is a schematic view of a chemical vapor depositionapparatus according to an exemplary embodiment of the present invention.

[0024]FIG. 3 is a flowchart illustrating a purge method for depositing asilicide according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0025] In the following detailed description, for purposes ofexplanation and not limitation, exemplary embodiments disclosingspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone having ordinary skill in the art having had the benefit of thepresent disclosure, that the present invention may be practiced in otherembodiments that depart from the specific details disclosed herein.Moreover, descriptions of well-known devices, methods and materials maybe omitted so as to not obscure the description of the presentinvention.

[0026] Briefly, the present invention relates to a silicide depositingmethod that illustratively includes depositing a silicide on a substrateand purging residual gases remaining from the depositing step by flowingair including H₂O (g), to thereby remove fumes caused by the residualgases.

[0027] In the depositing step, a tungsten silicide (WSi_(x)) isdeposited via a low-pressure chemical vapor deposition (LPCVD) method ata temperature illustratively in the range of approximately 500° C. toapproximately 800° C. and a pressure illustratively in the range ofapproximately 0.1 Torr to approximately 760 Torr, using tungstenhexafluoride (WF₆) and dichlorosilane (DCS) as reaction gases.

[0028] In the purging step, air including H₂O (g) may be used solely orbe mixed with oxygen (O₂) and at least one inert gas. The inert gasplays a role of preventing the silicide film coated on the substratefrom reacting to H₂O (g) or O₂ (g). The inert gases are illustratively,argon (Ar), nitrogen (N₂) or helium (He). It is preferred that eachpartial pressure of O₂ and H₂O (g) is illustratively approximately 10%or more in the gas mixture comprising air including H₂O (g), O₂ andinert gas. Further, when performing the purging step for removing fumes,air including H₂O (g), O₂ gas and inert gas may be successively suppliedto suppress the generation of particles.

[0029] In general, P-based and/or Cl-based residual gases absorbed onthe surface of the wafer after depositing the DCS react with H₂O (g)(and O₂ (g), if mixed with H₂O(g)) to form a stable compound. Thereby,these residual gases are advantageously desorbed from the surface of thewafer. Accordingly, if the wafer is left as it is in the atmosphereafter depositing the DCS, the residual gases generating the fumes reactwith H₂O (g) (and O₂ (g), if mixed with H₂O(g)) in the atmosphere, andare desorbed from the wafer surface.

[0030] As can be appreciated, since the P-based or Cl-based gases arevery noxious to the human body, to leave the wafer in the atmospherecauses the safety issues. Accordingly, if air including H₂O (g) (andO₂(g), if mixed with H₂O(g)) is supplied into the apparatus afterdepositing the DCS, the P-based and/or Cl-based residual gases absorbedon the wafer surface can be removed without causing the safety issues.At this time, in order to maximize the effect of removing fumes, theremoval of residual gases (cycle purging) can be carried out at apressure illustratively in the range of approximately 500 Torr toapproximately 760 Torr; more particularly in the range of approximately600 Torr to approximately 760 Torr.

[0031] Referring to FIG. 2, a CVD system according to an illustrativeembodiment of the present invention comprises first and second load-lockchambers 100 and 102, respectively; first and second processing chambers106 and 108, respectively, mounted on the first and second load-lockchambers 100 and 102; source lines (not shown), which are connected tothe first and second processing chambers 106 and 108 and supply reactiongases into the first and second processing chambers 106 and 108; ventline 16, which is connected with the first and second load-lock chambers100 and 102 and supplies an inert gas such as nitrogen or argon into thefirst and second load-lock chambers 100 and 102; and air purge lines 10which are connected to the first and second load-lock chambers 100 and102, and which supply air including H₂O (g) into the first and secondload-lock chambers 100 and 102 to purge residual gases in the first andsecond load-lock chambers 100 and 102.

[0032] According to the illustrative embodiment of the presentinvention, an O₂ gas line 12 is connected to air purge line 10. The airpurge line 10, the O₂ gas line 12 and the vent line 16 are connected toone another. Therefore, the gas mixture comprising the inert gassupplied through the vent line 16, the air including H₂O (g) and O₂ gascan be used as the purge gas.

[0033] Alternatively, the air including H₂O (g) may be used solely.Alternatively, the purge step may be carried out by successivelysupplying the air including H₂O (g), an O₂ gas, and an inert gas.

[0034] First, second and third valves 18, 22 and 26 and mass flowcontrollers (MFC) 20, which are operated by controllers (not shown) andregulate the flow and amount of gases, may be connected to the air purgeline 10, the O₂ gas line 12 and the vent line 16, respectively. Further,filters 24 for removing particles by filtering air and gases suppliedvia these lines may be connected to these lines, respectively.

[0035] According to the illustrative embodiment of the presentinvention, when residual gases in the first and second load-lockchambers 100 and 102 are purged out by supplying air into the first andsecond load-lock chambers 100 and 102 via the air purge lines 10, thepurged residual gases are exhausted to a vacuum pump 30 connected withvent lines 16. Further, in order to more quickly exhaust the purgedresidual gases, the air purge line 10 and the O₂ gas line 12 may beconnected with the vacuum pump 30, respectively.

[0036] In general, since air including H₂O (g) or O₂(g) should not besupplied into the first and second processing chambers 106 and 108 whenthe first and second processing chambers 106 and 108 maintain a vacuumstate, it is useful to perform the purge step by connecting the airpurge line 10 and the O₂ gas line 12 to the first and second load-lockchambers 100 and 102. Accordingly, the source line connected with thefirst and second processing chambers 106 and 108 and the air purge line10 are illustratively separated from each other.

[0037]FIG. 3 is a flowchart of a purge method for depositing a silicideaccording to an illustrative embodiment of the present invention.

[0038] Referring to FIG. 3, a substrate such as a silicon wafer is firstloaded in a load-lock chamber of a CVD system (S10). The substrate istransferred into a processing chamber (i.e., a reaction chamber) of theCVD system (S12), and then, WF₆ and DCS are introduced into the reactionchamber (S14). Using WF₆ and DCS as reaction gases, a tungsten silicide(WSi_(x)) is deposited on the substrate (S16). After transferring thesubstrate into the load-lock chamber (S18), the gas mixture comprisingair including H₂O (g), O₂(g) and an inert gas is introduced into theload-lock chamber, thereby purging residual gases absorbed on thesurface of the substrate (S20). Then, the substrate is removed from theCVD system (S22).

[0039] Hereinafter, a method of depositing a silicide according to anillustrative embodiment of the present invention is described, in moredetail, with reference to FIGS. 2 and 3.

[0040] Referring to FIGS. 2 and 3, a substrate such as a silicon waferis loaded into a cassette in the first load-lock chamber 100 or thesecond load-lock chamber 102 of the CVD system (S10). Illustratively,after loading the wafer into the first load-lock chamber 100, the firstload-lock chamber 100 is pumped down to the pressure of approximately200 m Torr. The first load-lock chamber 100 substantially maintains avacuum state.

[0041] Next, a slit valve (not shown) between the transfer module 104and the first load-lock chamber 100 is opened and the wafer in the firstload-lock chamber 100 is transferred into the first processing chamber106 or the second processing chamber 108 by the robot arm of thetransfer module 104 (S12).

[0042] If the wafer is transferred into the second processing chamber108, reaction gases, e.g., WF₆ and DCS are introduced into the secondprocessing chamber 108 (S14). By doing so, a tungsten silicide (WSi_(x))is deposited on the wafer by the reaction of WF₆ and DCS (S16).Illustratively, a LPCVD process is carried out at a pressureillustratively in the range of approximately 0.1 Torr to approximately760 Torr and a temperature illustratively in the range of approximately500° C. to approximately 800° C.

[0043] After the deposition is completed, the second processing chamber108 is pumped out to the pressure of approximately 200 m Torr, therebymaintaining the second processing chamber 108 at a high vacuum state.After the completion of this pumping step, a slit valve (not shown)between the transfer module 104 and the second processing chamber 108 isopened and then, the wafer is transferred into a cooling stage 110 bythe robot arm of the transfer module 104. After cooling the wafer at thecooling stage 110 illustratively for approximately 60 to approximately300 seconds, the slit valve between the transfer module 104 and thesecond load-lock chamber 102 is opened and then, the wafer istransferred into the load-lock chamber 102 (S18).

[0044] If all wafers are transferred into the cassette of the secondload-lock chamber 102 via the above-described steps, residual gasesabsorbed on the surface of the wafer after the DCS deposition aresubstantially removed by supplying only air including H₂O (g); or thegas mixture of air including H₂O(g), O₂(g), and inert gas, as the purgegas (S20). Then, the purged residual gases are exhausted to the vacuumpump 30 (S22). These steps are described presently.

[0045] The first, second and third valves 26, 22 and 18, which areconnected respectively to the air purge line 10, the O₂ gas line 12 andthe vent line 16, are successively opened to introduce the gas mixtureof air including H₂O gas, O₂ gas and inert gas into the second load-lockchamber 102. Then, after closing the valves of the air purge line 10 andthe O₂ gas line 12, the valve 18 of the vent line 16 and the valve 28 ofthe vacuum pump 30 are opened. By doing so, the residual gases purgedfrom the second load-lock chamber 102 are exhausted through the ventline 16 to the vacuum pump 30. At this time, if the vacuum pump 30 isalso connected with the air purge line 10 and the O₂ gas line 12, theresidual gases purged from the second load-lock chamber 102 can be morequickly exhausted.

[0046] Alternatively, in the purge step, the cycle purging may becarried out ten times or more at a pressure of illustratively in therange of approximately 500 Torr to approximately 760 Torr; moreparticularly in the range of approximately 600 Torr to approximately 760Torr. Thus, the residual gases absorbed on the wafer surface can be moreeffectively removed.

[0047] Then, after the purge step is completed, a vent gas such asnitrogen (N₂) or argon (Ar) is supplied through the vent line 16connected with the second load-lock chamber 102 until the pressure ofload-lock chamber 102 is approximately 760 Torr. As a result, the secondload-lock chamber 102 is vented and the wafers are removed from the CVDsystem.

[0048] Applicants have tested the deposition of suicide layer bychanging the deposition conditions and purge conditions.

[0049] The following table 1 shows exemplary experiment results obtainedfrom the various methods for removing the generation of fumes. TABLE 1Amount of No Deposition conditions and purge conditions fumes 1Deposition at 700° C. (wafer cooling for 60 sec) A 2 Deposition at 650°C. or less (cooling for 60 sec) B 3 Deposition at 700° C. cooling for300 sec B 4 Deposition at 700° C. cooling for 1200 sec B 5 Deposition at700° C. cooling for 300 sec B Load-lock cycle purging 10 times using N₂6 Deposition at 700° C. cooling for 300 sec C Load-lock cycle purging 10times using dry air 7 Deposition at 700° C. cooling for 300 sec DLoad-lock cycle purging 10 times at a pressure of approximately 600 Torrto approximately 760 Torr, using air including H₂O (g) 8 Deposition at700° C. cooling for 300 sec X Load-lock cycle purging 10 times at apressure of approximately 600 Torr to approximately 760 Torr, using airincluding H₂O (g)

[0050] Here, the amount of fumes was obtained using olfactoric sensesafter unloading the cassette from the load-lock chamber, whileapproaching on the wafer at a distance of about 5 cm to 15 cm. Thesymbol A indicates that substantial fumes are generated. The symbol Bindicates that the verification of fumes is possible. The symbol Dindicates that the verification of fumes is exceedingly difficult.

[0051] As shown in the table 1, according to the method of decreasingthe silicide deposition temperature (No. 1 and No. 2), the method ofincreasing the wafer cooling time (No. 3 and No. 4) or the method ofcycle-purging the load-lock chamber by using N₂ gas (No. 5), fumes arenot removed completely. Further, in the case of using dry air as thepurge gas (No. 6), fumes are not completely removed.

[0052] In the case of using air including H₂O (g) as in the presentinvention (No. 7 and No. 8), removing fumes is substantially moreeffective as compared to the other methods. However, as the pressure ofcycle-purge decreases to 500 Torr or less (No. 7), the removing effectis somewhat diminished.

[0053] Accordingly, in the case of performing cycle-purge at a pressureillustratively in the range of approximately 600 Torr to approximately760 Torr similar to the flow of atmosphere by using air including H₂O(g), removing the generation of fumes is most effective.

[0054] According to the present invention as described above, airincluding H₂O (g) is supplied to purge the residual gases remaining fromthe deposition of the silicide film, thereby removing the fumes causedby the residual gases. The inert gas and O₂ gas may be mixed with airincluding H₂O (g). In the purge step, the cycle-purge can be carried outat a pressure of illustratively in the range of approximately 600 Torrto approximately 760 Torr similar to the flow of atmosphere, therebymaximizing the removing effect of fumes.

[0055] While the present invention has been particularly shown anddescribed with reference to illustrative embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be effected without departing from the spirit and scope ofthe invention as defined by the appended claims and the legalequivalents thereof.

What is claimed is:
 1. A chemical vapor deposition method, comprising:(i) depositing a silicide on a substrate; and (ii) purging residualgases remaining from said depositing step by using air including H₂Ogas.
 2. The method as recited in claim 1, wherein in (i), said silicideis deposited using tungsten hexafluoride (WF₆) and dichlorosilane (DCS)as reaction gases.
 3. The method as recited in claim 1, wherein in (i),said silicide is deposited at a pressure in the range of approximately0.1 Torr to approximately 760 Torr and a temperature in the range ofapproximately 500° C. to approximately 800° C.
 4. The method as recitedin claim 1, wherein in (ii), said air including H₂O gas further includesO₂ gas and at least one inert gas.
 5. The method as recited in claim 4,wherein said at least one inert gas is selected from the groupconsisting essentially of argon (Ar), nitrogen (N₂), and helium (He). 6.The method as recited in claim 4, wherein a partial pressure of each ofsaid O₂ gas and said H₂O gas is approximately 10% or more.
 7. The methodas recited in claim 1, wherein the method further comprises: after (ii),purging said residual gases by successively flowing O₂ gas and at leastone inert gas.
 8. The method as recited in claim 1, wherein in (ii),said purging of said residual gases is carried out at a pressure in therange of approximately 500 Torr to approximately 760 Torr.
 9. A chemicalvapor deposition method, comprising: (i) loading a substrate in aload-lock chamber of a CVD system; (ii) transferring said substrate intoa processing chamber; (iii) depositing a silicide on said substrate insaid processing chamber; (iv) transferring said substrate into saidload-lock chamber; and (v) purging residual gases remaining from saiddepositing step by flowing air including H₂O (g) into said load-lockchamber.
 10. The method as recited in claim 9, wherein in (iii), WF₆ andDCS are introduced as reaction gases into said processing chamber. 11.The method as recited in claim 9, wherein in (v), said air including H₂Ogas further includes O₂ gas and at least one inert gas.
 12. The methodas recited in claim 11, wherein a partial pressure of each of said O₂gas and said H₂O gas is 10% or more.
 13. The method as recited in claim9, wherein in (v), said purging of said residual gases is carried out ata pressure in the range of approximately 500 Torr to approximately 760Torr.
 14. The method as recited in claim 9, the method furthercomprising: after (v), purging said residual gases by successivelyflowing O₂ gas and at least one inert gas.
 15. A chemical vapordeposition apparatus, comprising: a load-lock chamber; a processingchamber mounted on said load-lock chamber; a vent line connected withsaid load-lock chamber; and an air purge line connected with saidload-lock chamber, wherein said air purge line supplies air includingH₂O gas.
 16. The apparatus as recited in claim 15, wherein said airpurge line and said vent line are connected to each other.
 17. Theapparatus as recited in claim 16, further comprising a vacuum pumpconnected to said vent line.
 18. The apparatus as recited in claim 15,further comprising an O₂ gas line connected to said air purge line. 19.The apparatus as recited in claim 15, further comprising a filterconnected to said air purge line.
 20. The apparatus as recited in claim15, further comprising a vacuum pump connected to said air purge line.